It is common when making steel to take molten iron from a blast furnace, subject it to desulfurization, introduce it into a basic oxygen furnace to remove carbon, and to then continuously cast the resultant liquid product. In desulfurization pretreatment, a lance is lowered into the molten iron in the transfer ladle and a controlled amount of powdered reagents consisting typically of magnesium, lime and calcium carbide is injected through the lance into the molten iron. Sulfur impurities are thereby reacted into insoluble sulfides that collect in the slag which can then be raked off. As a practical matter, it is desired to complete the desulfurization process without undue delay, in order not to interrupt downstream processing. If there is an interruption in flow or plugging of materials and the ribbon of continuous cast material becomes broken, significant costs are involved to restart the ribbon. Therefore, it is essential that desulfurization continue without significant interruption. To help ensure uninterrupted desulfurization, dual port lances such as that described in U.S. Pat. No. 5,188,661 were introduced, followed by dual lance desulfurization stations, as described for example in U.S. Pat. No. 6,010,658. In state of the art desulfurization stations, a mixture of powdered magnesium and a carrier reagent, like for example powdered lime and/or calcium carbide, is injected through each of a pair of lances of a dual lance station, or through each port of a dual port lance, into the molten iron.
The powdered reagents are initially stored in separate “injectors” each including a pressurized storage vessel and a single outlet orifice (co-injection). Alternatively, depending on the metallurgical treatment requirements of some applications, it is not required to use separate “injectors” but instead a single injector (mono-injection) is used that injects a suitable reagent containing the components required for that particular treatment application. For yet other metallurgical treatment requirements of some applications, it is required to use a combination of separate “injectors” and single injectors (multiple-injection) to be able to inject the desired combination of reagents for the given application.
For the sake of clarity the following disclosures do concentrate on the co-injection process of lime reagent and magnesium reagent but it shall be understood that the same principles shall apply to the other injection processes and suitable reagents as well. Flow of powdered reagent through the injector outlet orifice may be governed by a variable orifice valve of the type disclosed in U.S. Pat. No. 5,108,075, or by a fixed orifice valve. If a fixed orifice valve is used, flow rates may be varied by varying the pressure in the vessel, or by changing the orifice. A shut-off valve is also provided upstream of the orifice valve for selectively stopping flow through the orifice valve, thereby allowing for maintenance of the orifice valve.
Initially, an inert gas under pressure, which is typically referred to as transport gas, will be introduced into a tube below the outlet orifice of the lime injector to initiate flow of the lime reagent. The transport gas will then flow to a location below the outlet orifice of the magnesium injector, so the powdered lime can pick up the magnesium reagent and transport it to a lance.
FIG. 1 is a schematic diagram of a dual-lance desulfurization station 10 of the prior art. Station 10 includes a first magnesium injector 2 having a magnesium supply vessel 12 and a first lime injector 4 having a lime supply vessel 14, each injector 2, 4 feeding material into a first supply pipe 16 through respective outlet orifices 18 and 20. First supply pipe 16 carries material, with the help of an inert pressurized transport gas, to a first lance 22 for injection into molten metal contained within ladle 24. Station 10 also includes a second magnesium injector 3 having a magnesium supply vessel 13 and a second lime injector 5 having a lime supply vessel 15, each injector 3, 5 feeding material into a second supply pipe 17 through respective outlet orifices 19 and 21. Material from second magnesium injector 3 and second lime injector 5 flows with the aid of pressurized transport gas through second supply pipe 17 to a second lance 23 for injection into the molten metal within ladle 24.
As may be understood, dual lance system 10 requires a pair of magnesium injectors 2, 3 and a pair of lime injectors 4, 5 in order to supply each of the dual injection lances 22, 23 with a controlled amount of a suitably proportioned mixture of magnesium and lime. A similar duplication of reagent injectors is necessary in the case of a single immersion lance having independent, dual exit ports injecting magnesium-lime mixture though each port.